How to composite output of multiple scenes?
EDIT: Just realized there is a typo in the Blender docs, should be composed or composited, not composted, in case that was causing any confusion.

What this means is that you have multiple different scenes in multiple different .blend files, and you render each scene individually. Then, whether it be in Blenders Video Sequence Editor or another software, you combine the short clips into a longer work.

For an animation, no the full world that the animation takes place in is almost never fully built. Take a film like Toy Story for example. The artists didn't build all the parts of the city in the same file and render the entire animation in one go. Instead, they would have broken it up into a bunch of smaller files of various locations, or the same locations with different characters, or at different points in the film, or any number of reasons. Managing something the size of a feature film in a single file would be by all accounts impossible.

A short animation would be much easier to manage as a whole in comparison, but that's relative. It will still be easier to link objects, materials, animations, etc. to multiple scenes and combine the renders later on.